A lithium secondary battery is a secondary battery having a structure in which lithium dissolves out as ions from a positive electrode at the time of charging and moves to a negative electrode to be stored therein, and conversely, the lithium ions return to the positive electrode from the negative electrode at the time of discharging. Since such a lithium secondary battery has features such as high energy density and a long life, it is widely used as a power supply for electric appliances such as a video camera, portable electronic devices such as a laptop computer and a mobile telephone, electric tools such as a power tool, and the like. Recently, the lithium secondary battery is also applied to a large-sized battery that is mounted in an electric vehicle (EV), a hybrid electric vehicle (HEV), and the like.
A lithium secondary battery of this kind is generally constituted of a positive electrode, a negative electrode, and an ion conducting layer inserted between both of the electrodes. As the ion conducting layer, a separator formed from a porous film of polyethylene, polypropylene, or the like, which is filled with a non-aqueous electrolyte solution, is generally used.
However, since a flammable organic electrolyte solution is used as described above, it is required to improve a structure and materials for preventing volatilization or leakage, and also, it is required to install a safety device for suppressing an increase in temperature at the time of a short circuit and to improve a structure and materials for preventing a short circuit.
In contrast, an all-solid-type lithium secondary battery does not require a flammable organic electrolyte solution. Therefore, simplification of safety devices can be attempted, and the battery can be excellent in terms of production cost or productivity. Also, the battery has a feature that the solid electrolyte can be laminated in series in a cell, and thus voltage increase can be promoted. Furthermore, in a solid electrolyte of this kind, since nothing but an ion moves, side reactions caused by movement of anions do not occur, and it is expected that this leads to improvement of safety and durability.
A solid electrolyte to be used in the all-solid-type lithium secondary battery is required to have high ionic conductivity as far as possible and to be stable chemically and electrochemically. For example, lithium halide, lithium nitride, lithium oxoate, derivatives of these compounds, and the like are known as candidate materials for the solid electrolyte.
However, a solid electrolyte and a positive electrode active material to be used in the all-solid-type lithium secondary battery have a problem in that a high resistant layer is formed by reacting each other, and thus the interfacial resistance becomes large. Therefore, proposals for improving the interface have been disclosed.
For example, in regard to a positive electrode active material that can be used for an all-solid-type lithium secondary battery, International Patent Publication No. WO 2007/004590 discloses that a LiNbO3 coating layer is formed on the surface of a positive electrode active material, and, by using the positive electrode active material, the output characteristics of the all-solid-type battery can be improved by interposing a lithium ion-conducting oxide layer between the interfaces of the positive electrode active material and a solid electrolyte.
Further, Japanese Patent Laid-Open No. 2015-179616 discloses an active material powder that has a coating layer containing LiNbO3 on the surface of the active material powder capable of absorbing and desorbing lithium ions at a potential of 4.5 or more based on Li.